1. Field of the Invention
The present invention relates to a high efficiency wind turbine, and more specifically, to a rotor speed controller for a variable speed wind turbine that adjusts the rotor speed based on observed and commanded parameters.
2. Description of Related Art
Wind turbines have received increased attention over the past couple of decades as an environmentally clean energy source that is not dependent on limited fossil fuels. Presently, at certain times during the summer in California, Pacific Gas & Electric obtains up to 8% of its power from wind. Wind turbines are typically clustered in large numbers that make up wind power generating plants. In addition to windy sites developed in California, many windy locations within the United States have been identified as having a potential for profitable wind power generating plants.
Most wind turbines currently operate at constant speed. The rotor within them drives a generator, such as an induction generator, at a constant rotational speed. Constant speed operation is required for a direct link with a constant frequency electrical grid such as the 60 Hz electrical grid common in the United States.
However, operation at a constant speed has many drawbacks, a significant one of which is a loss of efficiency. The efficiency loss is particularly apparent in gusty conditions in which the wind speed rapidly fluctuates. In order to maintain constant speed during an increasing wind, braking must be applied, either electrical, mechanical or both. During a decreasing wind, energy must be applied to maintain the constant speed. In either instance, some of the wind energy in the gust is wasted. Furthermore, braking induces structural stresses on the drive train that reduce reliability and hasten the onset of fatigue. In technical terms, constant-speed turbines are optimized for energy capture at a particular wind speed. Wind speeds above and below that point result in less-than-optimal energy capture.
Variable speed wind turbines have been proposed as a solution to the drawbacks of constant speed wind turbines. In principle, variable speed wind turbines can provide much more efficient energy conversion. A variable speed machine's rotor will speed up with higher winds, providing reduced torque and mechanically softer loads through the drivetrain. Therefore variable speed wind turbines are subject to less stress than constant speed turbines, requiring less stringent drive train design parameters. Variable speed turbines can be more reliable and provide cost savings over the constant speed type.
For optimum efficiency, the rotor speed of a variable speed wind turbine should be proportional to the wind speed. It is highly desirable if the rotor speed is a linear function of wind speed. For example, during periods of increasing wind speed, the rotor should increase speed to follow the wind speed; during periods of decreasing wind speed, the rotor should decrease its speed. If the winds are fluctuating very slowly, a variable turbine speed without a rotor control system will be able find its own efficient operating speed. However, in typical gusty conditions, in which the wind speed is fluctuating, a substantial time lag can occur between beginning of a gust, and the rotor response time required to increase or decrease rotor speed to respond to the gust. During this time lag, the wind turbine is not operating efficiently.
Therefore, it would be an advantage to provide a controller that maintains an optimum relationship between rotor speed and wind speed; i.e., rotor speed should precisely track wind speed. For greater efficiency, the controller should immediately increase rotor speed during increasing wind speed and immediately decrease rotor speed during decreasing wind speed. Such a controller should be stable as a control system. However, the controller should respond quickly, not sluggishly, as many very stable control systems do. Additionally, the control system should allow for physical limitations of the turbine, such as the maximum allowable torque on the drive train, aerodynamic thrust on the rotor and power in the electrical system.
In order to track wind speed, it is necessary to know the average wind speed over the area swept by the blades of the wind turbine. In an attempt to provide an estimate of average wind speed, anemometers have been installed close to the area swept by the blades. However, anemometers cannot accurately measure the average wind speed of interest. One reason is that they can measure wind speed at only one location, while wind speed may vary considerably over the disk swept by the blades. Multiple anemometers may be better able to provide an estimate of the average speed. However they are not cost effective and are not sufficiently accurate. For example, an anemometer may be affected by the varying wind flow blockage of the rotor. Furthermore, with multiple systems, reliability in the field becomes a concern.
As a substitute for wind speed information, other controllers have measured quantities such as output electrical power and rotor speed and controlled torque using these quantities. However, these quantities are subject to forces in addition to wind, and therefore provide only a very rough estimate of the wind speed. It would be an advantage if the wind speed could be measured accurately as an average over the area swept by the blades of the wind turbine. It would be a further advantage if the average wind speed could be predicted at a subsequent point in time, and if the rotor speed could be adjusted to quickly track the wind speed for efficient operation.